Contract Report EL-93-1
June 1993
US Army Corps
of Engineers®
Waterways Experiment
Station
Zebra Mussel Research Program
Use of Emersion as a Zebra Mussel
Control Method
by
Robert F. McMahon, Thomas A. Ussery, Michael Clarke
University of Texas at Arlington
Approved for Public Release; Distribution is Unlimited
Prepared for
Headquarters, U.S. Army Corps of Engineers
The contents of this report are not to be used for advertising,
publication, or promotional purposes. Citation of trade names
does not constitute an official endorsement or approval of the
use of such commercial products.
The findings of this report are not to be construed as an official
Department of the Army position, unless so designated by other
authorized documents.
PRINTED ON RECYCLED PAPER
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Station Cataloging-in-Publication
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McMahon, Robert F.
Use of emersion as a zebra mussel control method / by Robert F.
McMahon, Thomas A. Ussery, Michael Clarke ; prepared for U.S. Army
Corps of Engineers; monitoredby EnvironmentalLaboratory,U.S.
Army Engineer Waterways Experiment Station.
33p. : ill. ; 28cm. — (Contract report; EL-93-1)
Includes bibliographicalreferences.
1. Hydraulic structures— Maintenance and repair. 2. Zebra mussel —
Control. 3. Foulingorganisms. 4. Aquatic pests— Control. 1.Ussery,
Thomas A. Il. Clarke, Michael. 111.
United States. Army. Corps of Engineers. IV. U.S. Army Engineer Waterways Experiment Station. V. Zebra
Mussel Research Program (U.S.) Vi. Ttile. V1l.Series:Contract repofl
(U.S. Army Engineer Waterways Experiment Station) ; EL-93-1.
TA7 W34C no.EL-93-l
iv
Figure 4.
Emersion tolerance times of zebra mussels under
varying conditions of temperature and relative
humidity . . . . . . . . . . . . . . . . . . . . . . . . . . ...13
Figure 5.
Effects of temperature and relative humidity on the
emersion tolerance of zebra mussels . . . . . . . . . . . . . 15
Figure 6.
Emersion tolerance times of zebra mussels under
varying relative humidities at a lethal
temperature of35°C
. . . . . . . . . . . . . . . . . . . ...16
Figure 7.
Tolerance of emersion in subfreezing temperatures
by zebra mussels as separated individuals
or clustered groups . . . . . . . . . . . . . . . . . . . . . . .17
Griffiths, Kovalak, and Schloesser 1989; Clarke 1952; Shtegman 1986;
McMahon 1990, 1992; McMahon and Tsou 1990; Claudi and Ackerman
1992). The byssal holdfast prevents mussels from being dislodged even in
high-velocity flows. Veligers metamorphose into “postveliger” larvae
prior to settlement. Once settled, postveligers quickly transform into juvenile mussels. Postveligers can settle in such great numbers, and juveniles
have such rapid postsettlement growth rates, that mussels can rapidly form
dense mats many shells thick (4 to 12 in. 1 thick) (Mackie et al. 1989; Griffiths, Kovalak, and Schloesser 1989; McMahon 1990, 1992; McMahon
and Tsou 1990; Greenshields and Ridley 1957; Jenner 1983). Rapid
buildup of mussel encrustations can reduce maximum-sustainable flow
rates even in large-diameter piping (Clarke 1952, Lyakhov 1986). Recent
experiences with zebra mussel infestations in power stations, potable
water treatment plants, and industrial facilities drawing water from Lake
Erie suggest that zebra mussel fouling may develop more rapidly and become more severe in North American raw water facilities than has generally been reported to be the case in Europe (Grifflths, Kovalak, and
Schloesser 1989; LePage 1989; Great Lakes Sea Grant Network 1991;
Electric Power Research Institute 1991; Ontario Hydro 1992).
Asian clam (Corbicu/a j7unzinea) macrofouling has been projected to
cost the U.S. power industry alone over $1 billion annually (Isom 1986),
and zebra mussel macrofouling has been estimated to cost $2 billion over
the next decade in the Great Lakes alone (Roberts 199@). Thus, the spread
of zebra mussels throughout the inland waterways of the United States
and Canada will eventually lead to greatly increased macrofouling costs
unless efficacious, environmentally acceptable, and cost-effective control
technologies are developed and implemented in the immediate future.
Biological Basis for Zebra Mussel Macrofouling
Zebra mussels rarely exceed 5 cm in shell length, most specimens in
North American fresh waters being less than 3 cm long (Griffiths et al.
199 1). Newly settled juveniles are 0.2 to 0.3 mm in length (Mackie et al.
1989). Retention of a byssal holdfast in adults and the planktonic veliger
larva make zebra mussels a major macrofouling species. The byssal holdfast allows mussels to attach and grow on the walls of piping, tube sheets,
embayments, or any other hard-surfaced components in raw water systems
(McMahon 1990; McMahon and Tsou 1990). For settlement of the
postveliger (length = 160 to 290 #m), flow rates must be below 1.5 to
2.0 rn/sec (Jenner 1983; Lyakhov 1986), but once attached, individuals
can tolerate considerably higher velocities. Thus, if postveligers settle
during low-flow conditions (e.g., in redundant systems, during off-line or
1
To convert inches to meters, multiply by 0.0254.
Chapter 1 Biology and Ecology of the Zebra Mussel
low-flow periods), they are unlikely to be dislodged by later exposure to
higher flows. North American and European experience has been that
mussels can make byssal attachment to any firm-surfaced material including metal, concrete, stone, wood, cloth, nylon, plastic, fiberglass, vinyl,
glass, and the shells of other bivalves (Mackie et al. 1989, Ontario Ministry for the Environment 1989).
Zebra mussels have separate sexes with external fertilization. The temperature range for embryonic development and hatching is 12 to 24 ‘C
(54 to 75 ‘F). The veliger larva hatches from the egg (Mackie et al. 1989)
having a thin, bivalved shell (protoconch), a ciliated “velure” for swimming and feeding, and a rudimentary foot. It is 0.04 to 0.07 mm in diameter at hatching and reaches 0.16 to 0.29 mm as a settlement-competent
postveliger. The veliger shell has a straight hinge (i.e., shell valves do not
project above the shell hinge line) while the shell valves of the postveliger
are characterized by the presence of distinct “umbos” (i.e., a dorsal portion of the shell valves projecting above the hinge line). Only the postveliger is capable of settlement. After settlement and initial byssal
attachment, the umbonal postveliger shell rapidly transforms into the typically anteriorly pointed, mussel-shaped shell of the adult (marked by reduction of the anterior end of the shell) (Hopkins 1989).
Zebra mussel growth rates and life span are dependent on environmental conditions. Greatest growth occurs in habitats with elevated temperatures and high food levels (i.e., suspended algae and bacteria) (Griffiths
et al. 1991, Morton 1969, Mikheev 1986). Growth rate decreases with
water depth. Growth rate is stimulated at water velocities of 0.5 to
0.8 m/see, but it is reduced where flow exceeds 1.5 m/see (Mikheev
1986). Studies of North American zebra mussel populations suggest that
annual mortality rates are high, with few individuals surviving beyond
3 years of life. Thus, the majority of individuals in natural and fouling
populations are small and less than 2 years old (Griffiths et al. 1991).
The high reproductive rates (30,000 to 40,000 eggs per female per
year) and growth rates of zebra mussels allow populations to rapidly form
thick encrustations in natural habitats and raw water systems. Natural population densities of 5,000 to 30,000 mussels/m2 are not uncommon
(Mackie et al. 1989) with 114,000 mussels/m2 being reported in a lagoon
pond (Wiktor 1963). In raw water systems, where continuous flow and
abundance of hard substrata provide mussels with optimal conditions for
settlement and growth, even greater densities have been reported. Densities of 700,000 individuals/m2 occurred in the intake canal of a power station on Lake Erie (Griffiths, Kovalak, and Schloesser 1989), indicating
that raw water systems may be particularly susceptible to massive zebra
mussel infestations.
Chapter 1 Biology and Ecology of the Zebra Mussel
3
Characteristics
of Zebra Mussel Macrofouling
Zebra mussel veliger densities in intake water can range from 70 to
400,000 veligers/m3 or 0.27 to 1,516 veligers/gal* (Mackie et al. 1989),
leading to extremely high entrainment and fouling rates. Extensive entrainment of postveligers allows mussel infestations to rapidly develop.
Up to 45,000 to 700,000 mussels/m2 have been reported to settle in raw
water systems within a single spawning season (Ontario Ministry of the
Environment 1989; Szlauer 1974; Griffiths, Kovalak, and Schloesser
1989). The presence of microbial films and surface corrosion stimulate
postveliger settlement (Jenner 1983, Jenner and Janssen-Mommen 1992,
Lyakhov 1986) by reducing flow at the substratum surface to levels that
induce postveliger settlement even in systems with flow velocities above
those generally reported to inhibit settlement (Mackie et al. 1989,
Lyakhov 1986).
Mussel fouling in raw water systems can occur on any hard surface (including embayment walls, stationary trash racks, pump intake housings,
and other exposed surfaces) receiving adequate flow and oxygen concentration and not exposed to temperatures greater than 30 “C (86 “F)
(McMahon 1990). Mussels may even attach to traveling screens and not
be removed by high pressure spray wash systems, particularly if screens
periodically stand idle for periods greater than 24 hr. Mussel fouling has
also been reported to have nearly completely occluded flow across stationary primary screens (Kovalak 1990). Mussels infesting intake structures
can reach sizes or be sloughed off as clusters of shells (bound together by
byssus threads) which are large enough to foul small-diameter downstream components such as condenser and heat exchanger tubes, smalldiameter piping, and fire protection systems (McMahon 1990, McMahon
and Tsou 1990). Such blockage reduces efficiency and aggravates corrosion downstream of blockage points. Blockage can also result from large
mussel shells becoming lodged lengthwise across tube inlet openings,
from young mussels attaching directly to tube walls, and from byssally
bound clusters of mussel shells becoming lodged in tube inlets (McMahon
1990, McMahon and Tsou 1990). Tube blockage by clusters of individuals makes the openings of even relatively large-diameter tubing (> 1 in.)
prone to mussel fouling (McMahon 1990, McMahon and Tsou 1990). In
addition to flow restriction or blockage, accumulation of sediments and
reduction of oxygen concentrations within thick encrustations of mussels
can create conditions exacerbating corrosion of metallic piping (McMahon
1990).
Zebra mussels tend to accumulate in any area where discontinuities in
water flow make conditions optimal for settlement. Thus, even in highflow systems (velocity >1.5 to 2 nhec) not generally susceptible to mussel
1
To convert gallons to liters, multiply by 3.785.
Chapter 1 Biology and Ecology of the Zebra Mussel
fouling, postveliger settlement will occur in the immediate vicinity of pipe
joints, valve seats and plates, and joints of unequal pipe diameter where
low-flow refugia develop (Jenner and Janssen-Mommen 1992). It is this
capacity to settle on almost any available hard surface where flow conditions are appropriate that makes the zebra mussel a major macrofouling
organism within its natural European range and that is likely to make it
the most costly and damaging aquatic species ever introduced into North
American fresh waters (Roberts 1990).
Chapter 1 Biology and Ecology of the Zebra Mussel
5
2
Investigation of the
Efficacy of Emersion
as a Zebra Mussel
Macrofouling Control
Technology
Dewatering of mussel-infested structures and subsequent lethal exposure of mussels to air could be a highly efficacious zebra mussel control
strategy, particularly in raw water systems such as navigation locks and
water-intake structures that are designed to be periodically dewatered for
maintenance. Such structures can often sustain relatively heavy mussel
infestations. Thus, the structures could remain operational for extended
periods before needing to be dewatered to eradicate mussel fouling. Utilization of dewatering strategies to mitigate zebra mussel fouling in such
structures has the obvious advantage of eliminating chemical treatment,
thus minimizing environmental impact (this may be a major consideration
for control of mussel fouling in navigation locks) while being relatively
cost-effective. It would be cost-effective because it would require no
retrofitting of these systems and because dewatering treatment of mussel
infestations could be integrated into routine maintenance schedules.
In spite of the potential efficacy of dewatering and emersion as a
means to control zebra mussel macrofouling, only a minimal amount of literature exists regarding the mussel’s emersion tolerance. Indeed, only relatively anecdotal information regarding its response to emersion has been
reported in European literature (Alyakrinskaya 1978) without extensive effort to quantify the effects of either temperature or relative humidity on
emersion tolerance and/or evaporative water-loss rate. There, also, have
not been any reports on the tolerance of this species to freezing temperatures while emersed.
Based on literature reports (reviewed by McMahon 1991), available evidence indicates that zebra mussels are much less tolerant of emersion than
are the vast majority of freshwater bivalves, suggesting that dewatering
Chapter 2 Emersion as a Zebra Mussel Control Technology
could be utilized as a zebra mussel control strategy. In order to provide a
basis for the evaluation of dewatering as a zebra mussel control technology, the tolerance of zebra mussels to emersion under a wide range of
ambient temperatures and relative humidities was investigated. Temperatures tested ranged from subfreezing to above the mussel’s upper lethal
limit. Tolerance of emersion at temperatures above freezing was determined over a relative humidity range of <5 percent to >95 percent. Water10SSrates were determined throughout tolerated emersion periods, and
quantitative models allowing prediction of emersion tolerance under specific combinations of temperature and relative humidity were developed.
Methods
Zebra mussels were collected from the intake of a power station drawing water from Lake Erie, shipped overnight in air, and held in insulated,
cooled containers while transported to the University of Texas at Arlington. They were held in 75 gal of continually aerated, dechlorinated, City
of Arlington tap water in a refrigerated “living stream” holding tank at a
constant water temperature of 5 ‘C * 0.5 “C (41 ‘F).
Prior to experimental determination of emersion tolerance, adult individuals were randomly selected (shell length range = 11 to 30 mm),
marked by painting an identifying number on each shell, weighed wet to
the nearest 0.1 mg, and reimmersed for 24 hr. After 24 hr reimmersion,
they were emersed under varying conditions of temperature and relative
humidity. Samples of 60 mussels each were exposed to one of 20 temperature and relative humidity combinations in plastic desiccator chambers.
Relative humidity (RH) was maintained in the desiccators by saturated
salt solutions. Mussels were held above these solutions on stages of
0.5-cm wire mesh covered with 1-mm nylon mesh. The RH levels tested
were c 5 percent over silica gel desiccant, 33 percent RH over MgC12 .
6H20, 53 percent over Mg(N03)2 . 6H20, 75 percent over NaCl, and
>95 percent over distilled water (Byrne, McMahon, and Deitz 1988). Test
temperatures for each RH treatment were 5, 15, 25, and 35 ‘C (41, 59, 77,
and 95 ‘F) maintained at *0.2 ‘C in a refrigerated incubator.
Periodically, subsamples of six individuals were removed from each
desiccator, reweighed, and their viability tested by dehydration in dechlorinated City of Arlington tap water for 12 hr at room temperature (22 to
24 ‘C; 72 to 75 “F). Frequency of subsample removal was designed to include emersion durations ranging from 100 percent subsample survival to
100 percent subsample mortality. Lethal emersion times were estimated
as LT50 (estimated time for 50 percent sample mortality) and LTIOOvalues
(estimated time for 99.9 percent sample mortality) by probit analysis
(Bliss 1936) and time to first observation of 100 percent subsample mortality. The natural logarithms of lethal emersion time values were fitted to a
least squares multiple linear regressions against temperature and RH as independent variables.
Chapter 2 Emersion as a Zebra Mussel Control Technology
7
At the end of the recovery period, all subsampled individuals were
dried to constant weight at 90 “C (194 ‘F). Subtraction of dry-weight
values from initial-weight values yielded the total water content (total
water weight = corporal + extracorporal water (i.e., mantle cavity water)
weight). Subtraction of the wet weight after emersion from the initial wet
weight yielded the weight of water lost during the emersion period. Water
loss was expressed as a percentage of the total water weight in fully hydrated individuals just prior to emersion (Byrne, McMahon, and Dietz
1988). Computation of mean percent total water loss values for subsamples of individuals removed at different periods over the course of emersion at any one RH and temperature combination allowed evaluation of
cumulative water loss over the entire tolerated emersion period. In all
cases, water-loss values were computed only for individuals surviving a
particular emersion period.
Tolerance to freezing was determined for adult mussels held at 5 ‘C
(41 “F) prior to experimentation under conditions similar to those described above for emersion tolerance determinations. Subsamples of 10
adult mussels ranging in shell length from 8.5 to 34.0 mm where placed in
jacketed, 400-ml glass beakers in which subzero degree Celsius temperatures where maintained (5z0.1 ‘C) by circulation of water containing antifreeze from a Lauda K-2/R refrigerated constant-temperature circulator
whose cooling capacity was increased by insertion of a refrigerated coldprobe (Forma Scientific, model 8366). The bottom of the freezing chamber was covered with two layers of 1-mm nylon mesh to prevent freezing
of mussels’ shells to the vessel’s walls. Specimens were placed in the bottom of the chamber either as 10 separated individuals or as clusters of 10
individuals bound together in 1-mm nylon mesh held closed with a rubber
band. The chamber opening was closed with a rubber stopper penetrated
by a thermometer to record chamber temperature. The chamber temperature was first stabilized at experimental temperatures of either O, -1.5, -3,
-5, -7.5, or -10 “C (32, 29, 27, 23, 19, and 14 ‘C). The sample of mussels
(separated or clustered) was then quickly placed in the chamber, and the
chamber restoppered. After a predetermined exposure period, mussels
were removed from the chamber and allowed to recover for 12 hr in dechlorinated, City of Arlington tap water held at 5 ‘C (41 ‘F) in a refrigerated incubator. Duration of exposure of successive subsamples at any one
test temperature was increased until 100 percent sample mortality was
achieved in at least two consecutive subsamples or for a total 48-hr exposure if 100 percent sample mortality did not occur.
In both ernersion tolerance and freezing tolerance experiments, the viability of individuals in subsamples was estimated after recovery and dehydration by gently touching the posterior mantle edge and siphons of
individuals with the bristles of a small, fine brush. If this gentle, tactile
stimulation did not induce valve closure, mantle edges and siphons were
more strongly stimulated with a dissecting needle. If either gentle or
strong stimulation resulted in valve closure, the individual was considered
recovered. If strong stimulation did not elicit valve closure, the individual
was considered dead.
Chapter 2 Emersion as a Zebra Mussel Control Technology
Results
At test temperatures within the ambient temperature range of 5 to
25 ‘C (41 to 77 ‘F), water-loss rates were clearly correlated with both temperature and RH. Water-loss rates increased with increasing temperature
within any RH treatment and decreased with increasing RH within any
one temperature treatment (Figure 1). At 15 ‘C (59 “F) and 25 “C
(77 “F), water loss was continuous throughout tolerated emersion; however, at 5 ‘C, water-loss rates were greatly reduced after an initial 24-hr
period in which 35 to 40 percent of total water was lost. This initial high
rate of water loss appeared to be associated with expulsion of extracorporal mantle cavity water due to the extensive valve-gaping behavior displayed by specimens emersed at 5 “C in all tested RH. Tendency to gape
was less pronounced in specimens emersed at 15 and 25 ‘C, reducing the
relative rate of water loss in the early stages of emersion compared with
individuals emersed at 5 “C. In all RH treatments at the lethal temperature of 35 ‘C (95 ‘F), water-loss rate was initially high (=40 percent of
total water) within the first hour of emersion, again due to extensive valve
gaping. Thereafter, water-loss rates at 35 “C slowed at all RH, being
slower at 75 and >95 percent RH and faster at 53, 33, and <5 percent RH
(Figure 2).
From <5 to 75 percent RH, mean total water lost just prior to death at
5, 15, and 25 ‘C (41, 59, and 77 “F) ranged between 58 and 71 percent.
But at >95 percent RH, death occurred at lower levels of water loss (range
= 25 to 45 percent) in all three temperatures (Figure 3). Whether determined as LT50, LT100, or time to first 100 percent sample mortality values, the effects of temperature and RH on emersion tolerance within the
tolerated ambient temperature range of 5 to 25 ‘C were essentially similar
(Figure 4). All three measures of emersion tolerance generally increased
with increasing RH within any one temperature treatment and decreased
with increasing temperature within any one RH treatment (Figure 4). Maximum LT50 values occurred at >95 percent RH and ranged from 70 hr at
25 “C to 560 hr at 5 ‘C; corresponding LTIOOand time to first 100 percent
sample mortality ranges were 97 to 1,124 hr and 96 to 1,152 hr, respectively. Minimal LT50 values occurred at C5 percent RH and ranged from
42 hr at 25 ‘C to 170 hr at 5 “C, corresponding to LTloo and time to first
100 percent sample mortality ranges of 70 to 363 hr and 72 to 312 hr. respectively. The effects of relative humidity on emersion tolerance became
much more pronounced at lower temperatures (Figure 4).
The LT50, LT100, and time to 100 percent sample mortality values were
transformed into natural logarithms and fitted to least squares multiple
linear regression equations against temperature and RH as independent
variables. The resulting regression equations were highly significant
(P c 0.00001) allowing tight prediction of emersion tolerance under specific temperature-relative
humidity conditions. The regression equations
computed were:
Chapter 2 Emersion as a Zebra Mussel Control Technology
9
ao
1
I
HOURS
60 t
1
70
60
50
40
30
20
10
.—
r!
50
0
100
200
150
250
300
HOURS
‘o -
$
25°C
60 -
0
A
a
50 -
III
‘z
3
A
:
~
do
*
20
30 -
10
0“
o
20
40
60
80
HOURS
Figure 1.
Cumulative percent of total water (corporal + extracorporal)
lost over the duration of tolerated emersion (horizontal axis)
by zebra mussels, Dreissena po/yrnorpha, emersed at 5 “C
(41 0F),150C (590 F), and250C (770 F) under relative
humidities of <5% (0), 33% (0), 53% (A), 75% (0) , and
>95Y0 (v)
10
Chapter 2 Emersion as a Zebra Mussel Control Technology
80
35°C
O<5%RH
70
60
a
33%
RH
A
53% RH
O
75% RH
A
V > 95% RH
50
40
30
20
10
0
I
o
10
5
15
20
25
Hours of Emersion
Figure 2.
Cumulative percent of total water (corporal + extracorporal) lost over the
duration of tolerated emersion (horizontal axis) by zebra mussels,
Dreissena po/ymorpha, emersed at a lethal temperature of 35 “C (95 “F)
under relative humidities of <5, 33, 53, 75, and >95Y0
11
Chapter 2 Emersion as a Zebra Mussel Control Technology
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12
Chapter 2 Emersion as a Zebra Mussel Control Technology
,
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-“”
I
1000
5°C
600
t
< 5%
33%
53%
R.latiw
75%
> 95%
75%
> 95%
75%
> 95%
Humidity
15°c
1000
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800
*
5.
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mLT,,O
-,00%
.smlpl. Mommy
t
< 5%
33%
53%
R.latiw
1000
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a
%
4?
1
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DLTW
1
-LT,OO
-100%
600
400
200
Humidity
S.mpl.
Motility
I
-=5%
53%
33%
Relative Humidity
Figure 4.
Emersion tolerance times of zebra mussels, Dreissena po/ymorpha, under varying conditions of temperature and relative
humidity
Chapter2 Emersionas a Zebratvlus8elControlTechnology
13
in LT50 = 5.243- 0.074(”C) + 0.011 (% RH)
(r= 0.94, n = 15, F = 56.6)
in LTI ~. = 6.091- 0.087(”C) + 0.010(%
(r= 0.92, n = 15, F = 39.8)
RH)
In Time of First 100% Mortality = 5.917- 0.082(”C) + 0.010 (’%RH)
(r= 0.93, n = 15, F = 47.2)
where
r = correlation coefficient
n = number of individuals
F = F Test
The effects, predicted by these equations, of temperature and RH on
the emersion tolerance of zebra mussels are displayed in Figure 5. This
figure indicates that RH has a small impact on emersion tolerance at temperatures above 25 “C (77 “F); however, it has an increasingly greater impact on tolerance time as temperatures decline below 25 “C. The tolerated
period of emersion becomes greatly extended by elevated RH at temperatures below 10 “C (50 “F).
The effects
ofRH on thepattern
ofzebramusselemersiontolerance
at
a lethal
temperature
of35 “C (95‘F)(Jenner1983,Jennerand JanssenMommen 1992)was reversed
comparedwiththatrecordedwithinthenormal ambienttemperature
range(5to25 ‘C or41 to77 “F).Instead
of
increasing
withincreased
RH asoccurred
withinthenormalambienttemperature
range,LT50,LT100,and timetofirst
100percent
samplemortalityvaluesincreased
withdecreasing
RH being16.9,
41.0,and 30.0hrat
<5 percent
RH, respectively,
and decreasing
to9.6,14.9,14.0hr,respectively, at >95 percent RH (Figure 6).
Zebra mussels were not greatly tolerant of freezing temperatures.
When exposed as separate individuals, 100 percent mortality was recorded
at all test temperatures except O ‘C (32 ‘F) within 48 hr; LT50, LTIOO,and
time to first 100 percent sample mortality values were 13.5, 15.1, and 15.0
hr, respectively, at -1.5 “C (29 “F). All three values declined to less than
2 hr at -10 “C (14 ‘F) (Figure 7). Somewhat surprisingly, tolerance of
freezing temperatures increased at all test temperatures in clustered mussels. When clustered, mussels exposed to O and -1.5 “C displayed no mortality over the maximal 48-hr exposure duration. At -3.0 ‘C (27 “F),
LT50, LT100, and time to first 100 percent sample mortality values were
3.2, 24.0, and 7.0 hr. respectively, declining to 1.1, 3.7, and 2.0 hr, respectively, at -10 “C (Figure 7).
14
Chapter 2 Emersion as a Zebra Mussel Control Technology
1
01
0
5
1s
10
20
TEMPERATURE
25
30
25
20
25
20
(*C)
Soo
700
600
200
200
100
0
0
5
10
15
20
TEMPERATURE
(*C)
700
100% Sampl*
A20rt*lily
600
500
3W
200
100
1
0[
0
5
10
15
TEMPERATURE
Figure 5.
20
(“C)
Effects of temperature and relative humidity on the emersion tolerance of zebra mussels, Dreissena po/yrnorpha. Emersion
tolerance times estimated as time to 509’0sample mortality
(LT50), to 100% sample mortality (LTIOO), and to the first observation of 100% sample mortality (100% sample mortality)
over the tolerated ambient temperature range of O to 30 ‘C
(32 to 86 “F) at various indicated relative humidities. See
text for the multiple linear regression equations relating the
natural logarithm of emersion tolerance time values to temperature and relative humidity on which the data presented
in this figure are based
Chapter 2 Emersion as a Zebra Mussel Control Technology
15
30
-. I
1
Freeze Survival
25
OLTw
W
20
w 48 hours
- Separate
LTtm
-100%
Sample Mor&lity
15
10
5
o—
-10.0
-7.5
-5
Tomporaturo
0.0
(“C)
30
Freeze Survival
-1,5
-3.0
>48 hours
- Clustered
>48
1
ncurs
25
20
15
10
5
0
-10.0
-7.5
-5
Tomporaturo
Figure 7.
-3.0
-1.5
G.o
(“C)
Tolerance of emersion in subfreezing temperatures by zebra
mussels, Dreissena po/yrnorpha, as separated individuals or
clustered groups. Freeze tolerances were estimated as
tifYI(?S for so~o
S&impk
mortality
(LTso),
for 100~0 sample mOrtality (LTIOO), and for first observation of 1009’o sample mortality (lOOYOsample mortality) at temperatures of O, -1.5, -3,
-5, -7.5, and -10 ‘C (32, 29, 27, 23, 18.5, and 14 “F). The
notation ‘%48 hours” above sets of histograms in both panels
indicates that mussels emersed at those temperatures survived the entire 48-hr exposure period
Chapter 2 Emersion as a Zebra Mussel Control Technology
3
Discussion and
Conclusions
Discussion
Of the presently available nonchemical options for mitigation and/or
control of zebra mussel macrofouling, thermal treatment, nontoxic foulresistant coatings, disposable substrata, and manual removal techniques
have been most fully developed and implemented in Europe and North
America (Mackie et al. 1989, McMahon 1990, Jenner 1983,”Jenner and
Janssen-Mommen 1992). In contrast, mitigation and control technologies
centered on dewatering and exposure of mussel infestations to desiccation
or freezing temperatures have received no experimental attention. The
only information previously available regarding the desiccation resistance
of zebra mussels resulted from studies of their physiological capacity to
buffer the hemolymph (i.e., blood) pH while emersed (Alyakrinskaya
1978). In this work, mussels were exposed to air at room temperature (20
to 22 ‘C, 68 to 72 ‘F) without controlling RH. Under these conditions,
they survived no longer than 4 days.
Based on the limited data of Alyakrinskaya (1978), it has been recommended that before transportation between bodies of water, recreational
vessels be held out of water for at least 2 to 5 days in a hot, dry, sunny location in order to kill any zebra mussels attached to the hull. It was believed that this strategy could prevent inadvertent mussel dispersal into
new drainage systems (Ontario Ministry of Natural Resources 1990,
199 la, 1991b). The data presented in this paper suggest that even at near
zero RH at 25 ‘C (77 “F), an exposure duration of at least 3 days would be
required to kill 100 percent of adult mussels attached to a boat hull. At
25 ‘C, RH has little effect on lethal emersion times with 4 to 5 days required for 100 percent mortality, which is a level of emersion tolerance
similar to that reported by Alyakrinskaya (1978). However, as temperature declines below 25 ‘C, survival times of emersed mussels increase exponentially. Thus, emersion durations at temperatures below 15 “C
(59 ‘F), particularly at higher RH, may become too extended (> 10 days)
for practical control of mussel dispersal on trailered boats. Indeed, prolonged emersion tolerance of zebra mussels at temperatures S15 ‘C suggests
18
Chapter 3 Discussion and Conclusions
that dewatering may not be a practical method for controlling mussels in
power stations, waterways structures, and industrial facilities at any other
time than midsummer because of the extensive downtime required for mitigation by dewatering at lower air temperatures. At lower temperatures, dewatering would be a practical method for mitigation ofzebra mussel
fouling only in redundant systemswhere redundant components could be
alternately dewatered for periods long enough to achieve high mussel mortalities without affecting systemoperation.
It has been hypothesized that zebra mussels were introduced to the
Great Lakes by the dumping of ship ballast water containing veliger larvae or juveniles carried across the Atlantic Ocean from a European freshwater port (Hebert, Muncaster, and Mackie 1989; Mackie et al. 1989).
The exponential extension of emersion tolerance in zebra mussels with decreasing temperature and increasing RH suggests a second potential mode
of transoceanic transport. Zebra mussels are highly mobile. They readily
detach from the byssal holdfast and disperse away from areas of high density to colonize fresh substrata (McMahon 1990). This behavior could restilt in mussels rapidly colonizing the anchors and anchor chains of
vessels moored in a freshwater European port harboring an extensive
zebra mussel population. Under the cool, moist conditions prevalent in
the northern Atlantic ocean, mussels attached to an anchor chain could
readily survive emersion long enough to be transported into the Great
Lakes. Once the vessel moored in the Great Lakes, mussels could disperse from the anchor chain onto the surrounding substratum, forming a reproductive, founding population.
At 35 ‘C (95 ‘F), a temperature only 5 “C (9 ‘F) higher than the maximum tolerated temperature of 30 ‘C (86 ‘F) (Jenner and Janssen-Mommen
1992), zebra mussels had an LTIOOof less than 2 days regardless of RH.
Thus, even mild heating of air in dewatered structures could induce rapid
zebra mussel kills, making it a potent mussel-mitigation technology. Kills
of zebra mussels in dewatered pipes could also be greatly accelerated by
forcing hot air through them.
Zebra mussels appear to be very intolerant of prolonged emersion relative to other freshwater bivalve species. At 15 “C (59 ‘F) from <5 percent
to 75 percent RH, Asian clams (Corbicula j7umirtea) tolerated emersion
more than twice as long as zebra mussels (Dreissna polymorpha). However, at 25 “C (77 ‘F) from <5 percent to >95 percent RH, the emersion
tolerance of the two species was relatively similar (LT50 for C. jl!uminea =
71.4 to 78.2 hr (Byrne, McMahon, and Dietz 1988), LT50 for D. polymorpha = 42 to 70 hr). As occurred with zebra mussels in this study, increased RH had little effect on the emersion tolerance of Asian clams
above 25 “C, but greatly extended emersion tolerance at lower temperatures (Byrne, McMahon, and Dietz 1988). Both freshwater unionacean
and sphraeiid bivalves appear to be much more tolerant of emersion than
zebra mussels. Riverine and pond species of these two bivalve groups can
survive many months of emersion when exposed by receding water levels
during droughts and dry seasons (McMahon 1991). The very reduced
Chapter 3 Discussion and Conclusions
19
emersion tolerance of zebra mussels relative to unionacean and sphraeiid
bivalves suggests that they, like the emersion-intolerant C. j%aninea, are
on] y recent invaders of fresh waters (McMahon 1991). The frequent y and
duration of emersion experienced by freshwater bivalves are less predictable and more extended than those experienced by intertidal species. Neither zebra mussels nor Asianclamsappeartohavehad a longenough
evolutionary
history
infreshwaterstohavefully
evolvedthehighlevels
ofemersiontolerance
characteristic
ofmostunionaceanand sphraeiid
species
whichentered
freshwatersintheTriassic
andCretaceusperiods,
respectively (McMahon 1991).
Percent total water loss at death in zebra mussels ranged from 56 to
71 percent at <5 percent to 75 percent RH at all four test temperatures.
This range is similar to that of specimens of C. flurninea emerged under
similar conditions (=50 to 80 percent) (Byrne, McMahon, and Dietz 1988)
and falls within that reported for three species of freshwater unionaceans
more tolerant of emersion than zebra mussels (Holland 1991). The similarity of percent water-loss-at-death values for zebra mussels emersed in
5, 15, 25, and 35 “C (41, 59, 77, and 95 ‘F) over <5 percent to 75 percent
RH suggests that mussels emersed under these conditions died as a result
of lethal tissue desiccation. However, at >95 percent in all four test temperatures, percent water loss was considerably below that recorded at
lower RH values. The occurrence of mortality at lower than tolerated levels of desiccation suggests that death of emersed specimens at >95 percent
RH was not due to lethal tissue desiccation, but, rather, to some other
emersion-induced stress such as disruption of hemolymph acid-base balance, ammonia toxicity, or exhaustion of organic energy stores (McMahon
1991, Byrne and McMahon 1993).
At 5 (41), 15 (59), and 25 “C (77 “F), emersion tolerance decreased progressively with decreasing RH. In contrast, at 35 “C (95 ‘F), emersion
tolerance increased progressively with decreasing RH. At all test temperatures, the rate of evaporative water loss increased with decreasing RH,
suggesting that the more rapid moralities recorded at lower RH at test temperatures of 5, 15, and 25 “C were due to lethal desiccation. Thus, the increase in tolerance in specimens emersed in lower RH at 35 ‘C initially
appeared somewhat anomalous. However, as 35 “C was marginally above
the mussels’ upper lethal temperature limit, emersion under low RH appeared to allow mussels to maintain tissue temperatures below the longterm upper lethal temperature limit of 31 ‘C (88 ‘F) (Jenner 1983, Jenner
and Janssen-Mommen 1992) by evaporative cooling. In contrast, at
>95 percent RH, capacity for evaporative cooling appeared to be greatly
reduced as indicated by the near zero water-loss rates of emersed individuals after an initial rapid loss of mantle cavity water due to valve gaping
(Figure 3). Inability to evaporatively cool tissues at >95 percent RH appeared to result in very rapid mortality (LT50 c 10 hr). However, the
LTIOOfor mussels emersed at 35 “C and >95 percent RH was still approximately 75 times greater than that recorded in submerged individuals held
at that temperature (Jenner 1983, Jenner and Janssen-Mommen
1992),
20
Chapter 3 Discussion and Conclusions
suggesting that even under near 100 percent RH, evaporative cooling can
increase the tolerance of lethal temperatures in emersed individuals.
The rapid kills resulting when emersed mussels were exposed to air
temperatures only a few degrees above the species’ upper lethal temperature limit suggest that injection of heated air into dewatered structures
could greatly increase the effectiveness of dewatering as a zebra mussel
macrofouling control technology. Generation of heated air requires far
less energy than heated water, and the equipment for producing heated air
is readily available, highly mobile, and easily scaled to almost any size
raw water structure. Use of heated air would not require excessive piping
as would use of heated water or steam. In addition, venting of warmed air
into the atmosphere would have negligible environmental impact. Heated
air treatment may be particularly effective in small structures that are
readily dewatered such as pipe segments, heat exchanges, and gauges.
There have been no previous studies of the freezing tolerance of zebra
mussels or other freshwater bivalve species. Northern temperate intertidal
gastropod and bivalves, which withstand regular tidal emersion in freezing temperatures, tolerate air temperatures as low as -22 ‘C (-8 ‘F) and tissue temperatures as low as -10 to -6 “C (14 to 21 ‘F) (Newell 1979). In
contrast, zebra mussels showed little tolerance of emersion in freezing
temperatures, being intolerant of emersion at temperatures <-1.5 ‘C
(29 “F) when exposed as separated individuals and intolerant of s-3.0 ‘C
(27 “F) when emersed as clusters of individuals. The rapid mortality resulting from emersion at temperatures s-3 ‘C (LT1O. at -3 “C <24 hr and
at -10 ‘C <4 hr) indicates that dewatering of mussel-infested structures
during periods of freezing air temperatures could be a very efficacious,
cost-effective, environmentally acceptable mitigation technology in northern North America where freezing air temperatures are common during
winter months. Mitigation of zebra mussels by dewatering during freezing conditions may be a particularly effective control strategy in facilities
such as navigation locks which are not in service or have little operational
demand when the waterways on which they are located are frozen over.
This mitigation technique could also be utilized in raw water systems with
redundant components which could be alternately dewatered during freezing periods without affecting operations.
The increased freezing tolerance of zebra mussels when clustered suggests that longer periods of emersion in freezing temperatures will be required to achieve 100 percent mussel mortality in structures infested with
dense mussel encrustations many shells thick. The mechanism causing increased freezing tolerance in clustered mussels is unclear. It is unlikely to
result from differences in tissue temperatures or rate of tissue freezing in
clustered versus separated individuals because exposure times were many
times greater than required for tissue temperatures to equilibrate to test
temperatures. Research is presently under way to determine the actual tissue freezing temperatures of clustered versus separated mussels.
Chapter 3 Discussion and Conclusions
21
The results of this research indicate that reservoir drawdown could be a
potentially efficacious method to control mussels in source water habitats,
particularly if carried out in midsummer when elevated temperatures
greatly reduce the time required to achieve mortality or in midwinter
when ernersed mussels could be exposed to lethal subfreezing temperatures. Natural zebra mussel populations generally occur above the thermocline (Mackie et al. 1989), limiting them to shallow, nearshore habitats.
Limitation to shallow waters would allow a high proportion of a zebra
mussel population to be eradicated by relatively small reductions in water
level. Indeed, the tendency for juvenile mussels settling in shallow waler
(<1 m in depth) to rapidly migrate to depths >1 m may be an adaptive
emersion-avoidance behavior in a species with little tolerance of aerial
exposure or freezing temperatures.
Conclusions
Therearea wide variety
ofoptions
presently
available
formitigation
and control
ofzebramusselmacrofouling.
Theseoptions
include
chemicaland nonchemicalapproaches
and off-line
oron-line
application
strategies.Among chemicalcontrol
options,
chlorination
and a nonoxidizing
biocideformulated
asa combination
ofdimethylbenzyl
ammonium chlorideand dodecylguanidine
hydrochloride
aremost commonly usedinNorth
Americanraw waterfacilities.
However,a number ofothermolluscicides
areundergoingtesting
and field
demonstration
forefficacy
against
zebra
mussels.Thiswillincrease
thearsenal
ofchemicaltreatments
thatwillbe
available
tocontrol
fouling
by thisspecies
inthefuture.
Toxicpaints
and
coatings,”
particularly
thoseimpregnated
withcopperorzincsalts,
also
appeartohaveefficacy
inpreventing
zebramusselsettlement.
Among nonchemical zebra mussel control technologies, manual removal, line pigs, and thermal treatments have been most extensively utilized in North America. Promising nonchemical zebra mussel control
technologies that require future research and development include robotic
cleaning devices, dewatering and desiccation, nontoxic foul-release coat-”
ings, mechanical strainers, infiltration systems, exposure to anoxia or hypoxia, and disposable substrata.
The most efficacious strategy for mitigation and control of zebra mussels at a specific raw water installation will depend on the design and operational requirements of that facility; the quality, hydrography, and
physical characteristics of its source waters; and the environmental and
other regulatory constraints under which it operates. In any one system,
the treatment strategy utilized is likely to involve a combination of several
technologies designed to be efficacious, cost-effective, and environmentally acceptable under the specific conditions and regulatory restraints of
that system. Certainly, the rapid spread of zebra mussels in North America
is providing impetus for extensive research and development of improved
bivalve macrofouling controls, including appropriation of funding specificallyy
22
Chapter 3 Discussion and Conclusions